(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engrne.
(2) Description of the Related Art
As a measure taken against exhaust gas pollution and fuel consumption, a lean burn system has recently been developed. According to this lean burn system, a lean mixture sensor is provided for generating an analog current in proportion to the air-fuel mixture on the lean side in an exhaust pipe of an engine. Thus, the feedback of the air-fuel ratio of the engine can be controlled by using the analog output of the lean mixture sensor, thereby attaining an arbitrary air-fuel ratio on the lean side.
In such a lean burn system, when the throttle valve opening is smaller than a definite value, such as 50.degree., feedback control of the air-fuel ratio of the engine is carried out so that the air-fuel ratio is brought close to a target air-fuel ratio calculated in accordance with a predetermined parameter, such as the intake air pressure PM or the intake air amount Q. Note that such a target air-fuel ratio is also called a base air-fuel ratio. On the other hand, when the throttle valve opening is equal to or larger than the definite value, feedback control of the air-fuel ratio is stopped. Instead of this, power fuel increment control is carried out to obtain a power output air-fuel ratio of the engine.
That is, as illustrated in FIG. 1, when driving at a low altitude location, when the throttle valve opening TA becomes 50.degree., the base air-fuel ratio A/F is already considerably small, i.e., on the rich side as indicated by an arrow X.sub.1 in FIG. 1. Therefore, even when the air-fuel feedback control is changed to power fuel increment control or vice versa, the change .DELTA.A/F of the base air-fuel ratio A/F is relatively small. Therefore, the torque T changes relatively smoothly.
Note that the relationship of the torque T to the base air-fuel ratio A/F is illustrated in FIG. 3, and power fuel increment control is usually carried out at a base air-fuel ratio A/F of about 12 to 13.
However, when driving at a high altitude location, as illustrated in FIG. 2, even when the throttle valve opening TA becomes 50.degree., the base air-fuel ratio A/F is still at a high level, i.e., on the lean side, as indicated by an arrow X.sub.2. Therefore, when air-fuel ratio feedback control is changed to power fuel increment control or vice versa, the change .DELTA.A/F of the base air-fuel ratio A/F is relatively large. Therefore, the torque T rapidly changes, thus reducing drivability.